In a microwavable bowl, combine the corn starch, salt, dry mustard and pepper. Stir in milk. Add butter. Heat for 5 minutes. Stir in 1 3/4 cups cheese until melted then add pasta. Pour into slightly greased crock pot. Sprinkle with reserved cheese. Cover and heat on the high setting for 30 minutes. Then switch to low until ready to eat (if late) or keep on high 20 more minutes and then serve.